1. Field of the Invention
The present invention relates in general to a voltage controlled oscillator (hereinafter, referred to as a ‘VCO’), and more particularly, to a VCO generating multi-phase, multi-harmonic output signals.
2. Description of the Related Art
A voltage controlled oscillator (VCO) is an oscillator whose frequency is adjusted by controlling an applied voltage and changed capacitance of a variable capacitor.
VCOs are essential components in the architecture of all telecommunication systems. The VCO in modern data communication systems is employed as part of a phase locked loop (hereinafter, referred to as a ‘PLL’) that generates local oscillator (hereinafter, referred to as an ‘LO’) frequencies for an up and down conversion of a radio signal from/to a baseband of a transmitter and receiver. For example, according to the architecture of the transmitter-receiver, that is, single IF (Intermediate Frequency), dual IF, or direct conversion, a transmitter-receiver chip can include a certain number of VCO circuits for generating LO signals. To cope with spectral purity and phase noise of communication systems, each VCO is stabilized with the PLL. As for another problem in a dual-down conversion super-heterodyne radio, an input signal should be converted to a quadrature baseband signal at the last down-conversion operation. For this conversion, a quadrature phase LO signal (also called as a phase (I) and quadrature (Q) signal, or an IQ signal) is required. The IQ signal is generated by applying a poly-phase filter to outputs of a differential oscillator, or applying a divide-by-four circuit to a token ring structure, or employing various kinds of means such as coupled oscillators for example. These methods, however, not only have their own drawbacks and limits, but also make the transmitter and receiver more complicated than necessary. In the case of an RC poly-phase filter, it is known that the frequency with 90 degrees phase difference, caused by process deviation during the manufacture of integrated resistors, is considerably different from a design target frequency. Meanwhile, the divide-by-four circuit makes the structure of a transmitter and receiver more complicated. The conventional coupled oscillator also has many shortcomings, such as, its output power is not sufficient for a highest dynamic frequency or it has a very complicated coupled structure.
FIG. 1 illustrates a multiphase output oscillator using a logic inverter gate.
As shown in FIG. 1, according to the multiphase output oscillator disclosed in U.S. Pat. No. 5,592,126, a plurality of ring oscillators are serially coupled in a loop. None of the oscillators oscillates freely because each oscillator adjusts its successor all the way around the loop. A phase transition between two points in the loop is expressed as an integer fraction of 360 degrees, and is dependent on the number of oscillators. In the case of an integer multiple of four oscillators, the maximum phase transition is 90 degrees. Although a radio frequency in the multiphase output oscillator can be separated to a different phase, first and second harmonics are inseparable. Therefore, the disclosure can only be used as a quadrature generator, not as a harmonic generator. Another defect of the disclosure is that because the circuits are organized in a ring oscillator building block, without an additional circuit, sufficient RF outputs and satisfactory noise performance can not be obtained.
FIG. 2 illustrates two fixed-frequency oscillators coupled in a ring topology. Particularly, U.S. Pat. No. 6,188,292 discloses two interconnected oscillators where frequency variation is achieved by varying the coupling between the two oscillators. The coupling is varied by using a variable current or voltage source. However, this type of circuit can only be used as a quadrature generator, and is not appropriate for a harmonic generator.
FIG. 3A illustrates a related art wide-band or multi-band voltage controlled oscillator.
Particularly, U.S. Pat. No. 6,417,740 disclosed this type of the VCO, in which a pair of LC oscillator circuits are cross-coupled through a transconductance control circuit. Under the transconductance control, through NMOS varactor tuning (the varactors M13, M14, M15, and M16 controlled by a voltage Vcap) and current injection (circuits 340, 380 controlled by a voltage Vcon), the tuning range is from 900 MHz to 1.3 GHz, and from 1.3 GHz to 2.4 GHz. The quadrature output voltage is −10 dB, which corresponds to −18 dBm at the impedance of the load 50 ohm. This type of circuit may be effectively used with a quadrature oscillator for a wide-band receiver system, but is not appropriate for the harmonic generator.
FIG. 3B illustrates an embodiment of coupling for an LC-based VCO.
U.S. Pat. No. 6,492,877 disclosed this type of coupling, in which two LC VCO signals are coupled through couplers or buffers and coupling loops, and the two-stage coupling loop system provides in-phase and quadrature signals in two stages. However, neither simulation nor measurement data is given, and there is no way to find out signal magnitude and a maximum system frequency (speed). Although a detailed schematic circuit diagram is not provided, it would be sufficient to note that the buffers in the disclosure reduce the load capacity of each VCO, and presumably improve the “performance” of the oscillator (this is not mentioned). The disclosed coupling method is effective for generating quadrature phase signals, but cannot generate differential phase signals.